Contents I. Introduction
The widespread presence of distributed generators (DGs) and storage systems (DSSs) in electricity grids has increased the need of a full interaction between such systems and the main grid, for limiting the occurrence of critical situations in terms of grid stability and protection, power quality levels maintenance, personnel and equipment safety and so on. In this framework, an important issue is related to islanding detection. This is an operational condition where a portion of the grid is supplied by DGs/DSSs and it is decoupled from the main distribution network. Unintentional islanding is an unwanted condition, because it does not allow DSO to properly manage the power network and it can cause dangerous situations for both grid equipment, loads and people. For this reason the conventional anti-islanding protection is basically related to DGs disconnection from the grid when islanding occurs. This is also the approach of the current Standards [1]–[4]. Anti-islanding protection is implemented in DGs/DSSs interface protection system (IPS), so they can be promptly disconnected when the islanding condition is detected. In a more advanced smart grid management, an "intentional" islanding could be allowed, leading to the operation of power subsystems (and microgrids) decoupled from the main grid and allowing the possibility of a higher supply continuity for preferential loads or small energy districts. In this case islanding detection is still important, for enabling voltage and frequency control in the supplied subsystem. In recent times the islanding detection issue has been debated in literature and several methods have been proposed to increase islanding detection capabilities especially in "non detection zone" (NDZ) situations [5]–[8]. This is a condition in which islanding can be undetected, because of small variations of voltage and frequency values from grid-connected to islanded condition. To limit this problems, more or less sophisticated approaches and algorithms have been proposed, all having their own strengths and weaknesses in terms of effectiveness, disturbances injection, complexity and so on. In this framework, the authors have proposed a hybrid method, which combines communications with the main grid and some local measurements, complementing the traditional voltage and frequency measurements and reducing the NDZ [9]. The main advantage of the proposed solution is that it starts from current Standards requirements (basically local measurements) and it improves the islanding detection effectiveness, even in NDZ situations, thanks to the communications with the main grid. This allows to obtain and enhanced solution which can be implemented in current measurement and communication platforms used for IPS and smart metering, with limited increase in hardware and firmware complexity (i.e. memory capability, computational costs, algorithms efficiency etc.)
